Re: acm_port_shutdown hangs for 30 seconds

[Oliver Neukum <oneukum@suse.com>]

On 12.03.22 14:58, Jookia wrote:
> Hello there,
> 
> I've been banging my head against this issue over the years but sat down
> and started to debug it today.
> 
> When I try to quit GNU screen, minicom or another serial program it
> sometimes hangs for around 30 seconds.
> 
> To reproduce I do this:
> 
> 1. Connect an Arduino Micro with stock LED blink firmware
> 2. Run 'screen /dev/ttyACM1 9600'
> 3. Type some letters (no response from the board is given)
> 4. Quit in some way
> 
> If I skip step 3 (typing letters), the hang does not happen.

You have something in the buffer, which the tty layer will try to send.

> In userspace the hang happens at a call to close() on the TTY, and using

The tty layer is supposed to wait a defined amount of time
if a tty needs to drain.
This amount (among many other things) is traditionally
set with the TIOCSSERIAL ioctl()

> perf and ftrace it looks to be spending a lot of time poisoning urbs in
> acm_port_shutdown.

That is a bit odd.

> - Is this a bug?

Maybe, but unlikely. If and only it takes much more time than the
termios say.

> - Can I reduce the timeout somehow?

ioctl() TIOCSSERIAL. If that does not work,
it's a bug.

	HTH
		Oliver